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My HVAC system is 2 zones, hooked up to an old (circa 1984) Mastertrol zone controller, and Trol-A-Temp dampers.

Attached to the panel is a "system selector switch". Basically, its a knob switch with 4 positions: heat/off/fan/cool. I manually change the setting based on time of the year (if i need heating or cooling). Wires leave this box and connect to the system selector switch posts on the zone controller (MABS-V3, Mastertrol Mark V). However, this seems to prevent me from ever being able to run the fan directly from a thermostat.

Since I cant seem to find any information on the zone controller (nor who installed it originally), I was wondering if anyone knew anything about it, and if there is a way to bypass the system selector switch so I can take advantage of newer t-stats (I want to be able to run the fan all day, or at least intermittent thru the day. we're getting areas in the house with large temp differentials and i was hoping running the fan would smooth them out).

Any information would be greatly appreciated. (BTW, I talked to Honeywell, and although they said its not possible, they admitted they werent sure because the setup is so old).

They are probably right since in your set up the call for heat or cool is what actuates the dampers. You may be able to adjust the minimum damper positions to say 15 or 20% (if that is an option) and run the fan continuously, granted you will not get the best airflow but its better than none.

I was thinking about that myself, but the only way to activate the fan is to set the knob to fan, or there is a switch under the know which I can set from Auto to On. However, in both cases, it prevents the t-stats from operating the furnace. So, with the fan on using either of those methods, I will never get heat until I turn the fan off.

I guess Im stuck for now. Maybe Ill re-visit the idea if I ever need anything in the system replaced (furnace is only 5 years old, so it will be a while ).

You would have to replace the zone panel. We have an office building with 2 systems. All 4 dampers worked but he wanted modern stats so we installed a new panel which would work with a powered open/powered closed damper and he got the control he wanted without changing everything.